Lanthanide-based catalysts are used to prepare high cis-1,4-polybutadiene. These catalyst systems are typically formed by combining a lanthanide salt, an organoaluminum compound, and a source of halogen. They typically give polymers having a cis-1,4-linkage content of greater than 93% cis microstructure.
Although these catalyst systems yield polymers having desirable high cis-1,4-microstructure, the polymers have a relatively high molecular weight distribution because the molecular weight of the polymers cannot be easily controlled. Attempts have been made to control the molecular weight of the polymers produced with these catalyst systems. For example, the aluminum to lanthanide ratio has been manipulated to affect the degree of polymerization and the polymer molecular weight. This method, however, has not proved successful because the molecular weight distribution cannot likewise be controlled.
Others have employed a silicon halide or organosilicon halide as the source of halogen, and then aged the catalyst system for at least 5 hours prior to contacting with the conjugated dienes to be polymerized. This approach, however, increases the manufacturing cost of the polymer.
There exists a need for an economical continuous polymerization process for producing conjugated diene polymers having a high cis-1,4-linkage content and narrow molecular weight distribution.